Silver halide is only inherently sensitive to ultraviolet ("UV") and blue light. It is well known that sensitizing dyes can be used to extend this sensitivity to the green and red regions. In some cases it is also advantageous to extend this sensitivity into the infrared region, for example to provide improved visual contrast in aerial photography. It has been reported that aerial films that are sensitized to infrared light (that is wavelengths beyond 700 nm) display better tonal separation (for example, sharp delineation of river bands, differentiation between clean and polluted surface water, and the like), better haze interpretation rendering the film of greater utility under poor weather conditions, and better performance during periods of restricted light (for example, early morning and late afternoon). For this application a sensitization maximum in the region of 720 to 730 nm is particularly desirable.
The advent of solid state diodes that emit in the infrared has also expanded the useful applications of infrared-sensitive photographic elements. These include making prints from digital information such as from computer assisted tomography scanners, various graphic arts products, and infrared-sensitive false color-sensitized photographic materials as described in U.S. Pat. No. 4,619,892.
Infrared sensitivity can be imparted by dicarbocyanine or tricarbocyanine dyes adsorbed to silver halide emulsions in the monomeric state. To encourage monomeric adsorption, deaggregants are frequently employed (for example, U.S. Pat. No. 5,108,882). However, at higher dye levels, which could provide-increased infrared speed, the deaggregants are unable to maintain the monomeric adsorbed dye state and increased infrared speeds are not obtained. Dyes which can form adsorbed long wavelength aggregates (sometimes called J aggregates) in the infrared are inherently capable of providing higher photographic speeds since higher dye levels enhance the aggregation process.
J-aggregating infrared dyes have a number of additional potential advantages over monomeric infrared sensitizers (see EP 0 531 759). J-aggregates have relatively narrow half-band widths which can result in better color separation and/or less safelight sensitivity when used in combination with emulsions sensitized at other wavelengths. It is also generally recognized that dye instability correlates with the length of the conjugated chain in cyanine dyes. Thus J-aggregating dicarbocyanine infrared dyes should be more stable than tricarbocyanine monomeric infrared sensitizers.
J-aggregating infrared dyes are rare. Benzoquinoline carbocyanine dyes combined with other carbocyanine dyes have been alleged to provide high infrared sensitivities (U.S. Pat. No. 3,615,634) but such benzoquinoline dyes suffer the disadvantages of being derived from the carcinogen B naphthylamine and having a peak sensitivity which is sensitive to emulsion morphology. On some emulsions, the peak sensitivity is close to 700 nm, which results in low infrared sensitivity. EP 0531 759 describes certain dicarbocyanine dyes which are stated to provide narrow J band sensitization peaking at wavelengths longer than 730 nm. However, such dyes do not always give acceptable infrared sensitization.
It would be desirable then, to have an infrared sensitizing dye which forms a J aggregate on various types of silver halide emulsions and therefore is not be used with deaggregating compounds.